Respiration systems are, for example, closed-circuit respirators, which are defined as respirators independent from the ambient atmosphere, and in which respiration gas circulates. They are used where harmful effects due to toxic impurities contained in the breathing air or oxygen deficiency must be expected. The freely portable closed-circuit respirators supply the user of the device with respiration gas, which is carried along in the device, generated and purified. The maximum service life of the devices varies and depends on the quantity of respiration gas being carried along or generated in the device as well as the consumption of air by the user of the device. The oxygen reserve in the device is carried along in these devices either as pressurized oxygen or as chemical oxygen (e.g., potassium superoxide). The breathing air exhaled by the user of the device flows in closed-circuit devices into a regenerating cartridge, in which the carbon dioxide (CO2) contained in the respiration gas is bound chemically. Among other things, water and a considerable quantity of reaction heat (approximately 113 kJ/mol) are generated in the process.
Exhaled CO2 is bound by means of a chemical reaction in another application. If potassium superoxide is used as the reactant, this reacts with the moisture exhaled simultaneously with the CO2 into oxygen, potassium hydroxide and potassium carbonate. Heat (169 kJ/mol) is likewise formed besides oxygen. Contrary to the regenerating cartridge, this process cannot be interrupted. Further hydroxides are also known, which may likewise be considered for use for binding CO2 from the exhaled air. H2O and heat are likewise formed in these reactions.
Respirators that are used as anesthesia apparatuses in medical engineering use carbon dioxide absorbers to remove the carbon dioxide exhaled by the patient from the breathing gas. Various designs of such absorbers are available. Disposable absorbers are known, which are disposed of completely after use, and refillable devices are known. The absorbent (breathing lime) is filled as a bulk material into these; the absorbent is disposed of after use and is replaced with fresh breathing lime.
It is of great significance in all applications to provide an indicator for the consumption. The term “consumption” will be used hereinafter in a general sense and shall also cover variables derived therefrom, e.g., the residual capacity of the reacting material, i.e., the complement of the quantity of material consumed.
A color indicator, which signals the exhaustion of the breathing lime on the basis of the reaction moisture formed by a change in color, is added to the breathing lime in prior-art CO2 absorbers used in medical engineering. The color change is read visually, but the limit of the color change often cannot be recognized unambiguously and the absorbers are therefore usually replaced at fixed intervals, regardless of whether or not their capacity has already been fully exhausted. An additional drawback of the color indicators is their reversibility, i.e., the color indicator returns to its original state after drying of the consumed breathing lime. It is therefore impossible to make a distinction between fresh and consumed (dried) breathing lime without a doubt. In addition, the odor and taste of the color indicator added are often felt to be at least unpleasant and a nuisance.
A respiration system of the type described in the introduction is known, for example, from DE 10 2005 015 275 A1, in which a method for determining the residual capacity of chemical canisters for use in oxygen-generating closed-circuit respirators is described. The number of breaths, the pressure level and the inhalation temperature are determined according to this patent, and the current breathing air consumption is determined from this and subtracted from the total capacity.
A respiration system, which recognizes the passage of the heat front generated during the CO2 absorption via an array of temperature sensors and makes it possible to obtain information from this on the state of consumption of the breathing lime absorber, is described in U.S. Pat. No. 6,618,687.
A corresponding electronic system is needed in both systems, and a distinction between consumed and unconsumed material is not possible or is possible with a considerable effort only, for example, after an intermediate storage. In addition, cabled temperature sensors are led through housing walls in as gas-tight a manner as possible in order to guarantee the operation of the devices.
A memory chip, on which the current state of consumption is calculated from the operating data of the respiration system, stored and updated, is proposed in DE 10 2005 026 838 B3.